Giardia lamblia, a CDC category B priority pathogen, causes one of the most common protozoal infections of the human intestine, and is an important cause of diarrheal disease with hundreds of millions of annual cases worldwide. Infection can be initiated by ingestion of fewer than ten cysts, making G. lamblia highly contagious and a credible threat to the safety of public water supplies and health. Trophozoites, the disease- causing forms of the parasite, colonize the lumen of the small intestine and attach to the epithelium. Symptomatic infection is characterized by diarrhea, epigastric pain, nausea, and vomiting, which may lead to malabsorption and malnutrition, especially in children. Infection is typically self-limiting, indicating that effective immune defenses exist. Despite the clinical importance of giardiasis, no preventive medical strategies are available. A crude veterinary vaccine has been developed and licensed for cats and dogs, but it is only modestly effective. A defined human vaccine remains unrealized. In prior and new preliminary studies, we have identified 19 conserved G. lamblia candidate antigens with minimal homology to human proteins. Preliminary immunizations with two of these proteins, a1-giardin and a-enolase, significantly protected against giardiasis in murine and gerbil infection models. The proposed exploratory studies have the overall objective to identify new protective antigens in G. lamblia that can ultimately be used in a safe and effective vaccine in humans. In Aim 1, we will determine the capacity of conserved antigen candidates to confer immune protection in mice against a representative G. lamblia strain. We will focus on a matched (homologous) system of immunizing antigen and infecting strain to minimize confounding variables related to potential antigen variants. For protective antigens, we will then begin to define the mechanisms of immune protection by testing mice deficient for key effectors of mucosal immunity against giardiasis, including B cells, IgA, and CD4 T cells. In Aim 2, we will determine immunization-induced cross-protection in gerbil models of giardiasis, and will define consensus antigens that can protect against infection with diverse G. lamblia strains. Gerbils can be more readily infected than mice with a wide range of strains, making them well suited for these studies. We will also begin to explore novel nanotechnology solutions for mucosal delivery of a protein antigen-based mucosal vaccine. Together, the proposed exploratory studies are important and innovative, because they will functionally evaluate the protective potential of conserved antigens against a common intestinal infection without a current human vaccine. Furthermore, novel nanotechnology approaches will be explored for the effective mucosal delivery of a protein antigen-based vaccine against an important lumen-dwelling intestinal parasite that can serve as a model for other enteric pathogens.